1. Field of the Invention
The present invention broadly relates to a system in which a tape-like member such as a recording medium (referred to as "tape" hereinafter) is made to run between two reels in accordance with one of a plurality of tape running modes having different tape-running speeds. More particularly, the present invention is concerned with a control apparatus for controlling the operation of the reel motors in such a system, in accordance with the selected tape running mode.
2. Description of the Prior Art
A recording/playback system has been known in which a tape runs between two reels, i.e., a supply reel and a take-up reel, wherein the reel support for the supply reel and the take-up reel are driven by independent driving motors. This type of tape driving mechanism is generally referred to as "direct drive tape-running mechanism". In this type of mechanism, the rotation speeds of the reels are controlled in accordance with control signals which are produced by a computing circuit in response to signals which represent the periods of rotation of respective reels.
When the tape is to be stopped, the driving motor for the take-up reel support or a capstan motor is stopped, while the supply reel support driving motor is reversed with a predetermined braking torque, so as to tension the tape.
This known tape driving mechanism has the following problem, particularly when the tape running mode is changed. Namely, the control of the driving motors upon detection of the periods of rotation of the motors is conducted even when the running mode is being changed, so that the output torques of the motor may be changed drastically, with the result that the tension on the tape is abruptly changed, thus adversely affecting the tape.
Another problem encountered by the conventional mechanism is attributable to the fact that the braking torque is not controlled in accordance with the instant conditions of the reels when the running tape is to be stopped. Namely, in the conventional mechanism, the level of the reversing or braking torque is maintained constant regardless of the state of rotation of the reel, although the state of rotation of the reel is momentarily changed according to the amount of the tape wound on the reel. This may cause the reel to be braked abruptly, with the result that the tape is stretched or slacked due to the inertia of the reel or the impact produced by the braking. This problem causes an impediment to the current demand for making the tape thinner which is necessary for attaining longer recording or playing time.
The change in the tension of the tape also causes a change in the contact between the tape and the recording and reproducing heads, which causes wear of these heads.
In general, the recording/playback apparatus of the kind described is required to control the reel motors to provide different tape speeds, as in the case in a change-over from the fast forward (FF) mode to the ordinary playback (PLAY) mode. It is also required that the tape tension be maintained constant regardless of the winding radius, i.e., the radius of the tape wound on the reel, which increases or decreases momentarily. To cope with these demands, it is necessary to perform a complicated computation, in order to optimally control the driving torque of the take-up reel motor in accordance with changes in operating conditions. Hitherto, the torque control of the reel motor has been carried out by varying the pulse width of the reel motor driving signal, such that a greater motor output torque is obtained as the pulse width of the motor driving signal is increased. This control method, however, requires an impractically long computing time, because a series of complicated computations has to be performed from the beginning, even in response to a slight change in operating conditions. It is to be noted also that the pulses have to be counted from the beginning for the purpose of setting the pulse width. During the counting of the pulses, all other operations are prohibited.